FIG. 4 is a block diagram showing an example of a conventional air conditioner 1. In the air conditioner 1 of this type, power is supplied from an original power source 2 on the outside to an outdoor unit 3 and, in addition, the power is branched in the outdoor unit 3 and supplied also to an indoor unit 4.
In this case, the power supply from the original power source 2 to the outdoor unit 3 and the power supply branched from the outdoor unit 3 to the indoor unit 4 are performed via respectively separated power source lines 5a, 5b, 6a and 6b. 
Reference numeral 7 denotes a signal line and, a bundle of electric lines obtained by bundling the signal line 7 and the power source lines 6a and 6b is called indoor/outdoor lines Lin. Reference numeral 9 indicates a noise generating source such as an inverter or the like in the outdoor unit 3 and reference numerals 10 and 11 denote a transmission circuit and a reception circuit in the outdoor unit 3. The transmission circuit 10 and the reception circuit 11 are connected to the power source lines 5a and 5b via power source lines 8a and 8b and, while receiving the supply of power, transmit/receive signals to/from the indoor unit 4 via the signal line 7. The power source lines 8a and 8b for supplying the power to the transmission circuit 10 and reception circuit 11 are branched and connected to the power source lines 6a and 6b of the indoor/outdoor lines Lin and supply power also to the indoor unit 4.
In the air conditioner 1, the noise generating source 9 exists as described above. Generally, it is requested to design the voltage level of each of noise 12 which is superposed in the power source lines 5a and 5b and noise 13 which is superposed in the power source lines 6a and 6b so as to be within a predetermined specification.
Conventionally, a first noise filter 15 is provided for the power source lines 5a and 5b extending between electrical parts in the outdoor unit 3 such as the noise generating source 9, transmission circuit 10, and reception circuit 11 and the original power source 2 and, a second noise filter 16 different from the first noise filter 15 is provided for the indoor/outdoor lines Lin extending between the electrical parts 9, 10 and 11 and the indoor unit 4.
As described above, when the different noise filters 15 and 16 are provided for the power source lines 5a and 5b between the original power source 2 and the outdoor unit 3 and for the indoor/outdoor lines Lin between the outdoor unit 3 and the indoor unit 4, a problem occurs such that even when the noise filters 15 and 16 independently try to eliminate the noises 12 and 13, the noise filters 15 and 16 cannot sufficiently eliminate the noises 12 and 13, respectively, due to insufficient properties of the noise filters 15 and 16 and the like.